In marine operations involving subsea work, it is often necessary to deploy a heavy structure, vehicle or piece of equipment in rough seas. Traditional deployment methods use various types of heavy cranes, heave compensation systems, reinforced cages/protective systems, “A” frames, moon pools and lowering decks. The heavy structure being deployed is typically mechanically coupled (directly or indirectly) to the heavy vessel deploying it, as the load is lowered into and retrieved from the water. The various deployment and retrieval systems attempt to resolve the fundamental problem of matching, or minimizing, the relative motion of two spatially separate but mechanically linked masses in a dynamic environment such as at the sea surface. Entry of the load into and exit of the load from a turbulent “splash zone” at and near the water surface can be treacherous and may damage the load or the launch/retrieval vessel, and may as well produce unsafe conditions for personnel and equipment in the area.
In one exemplary launch and recovery system disclosed in PCT International Patent Publication WO 2013/072690, a subsea payload is lifted from and deployed to an undersea location using a lift line supported by a heave-compensating winch on a surface vessel. Movement of a submersible latch unit is controlled by means of on-board thrusters. Vessel and crew time are expensive, and the expense incurred as a result of launch and retrieval vessel and crew requirements may limit the frequency of launch and retrieval operations, particularly in remote locations.
One system that avoids mechanically joining heavy dissimilar objects (such as a launch vessel and a submersible vessel or payload) at the splash zone places the vehicle or payload on a “sinking barge” and tows the barge/payload to a dive location, where both the barge and the payload are sunk, as a unit, under controlled conditions, generally using an active depth control system. The payload/vehicle is deployed by releasing it from the barge transport component at a desired depth where the sea conditions are steady and manageable. The few existing systems that use this type of subsurface launch and retrieval approach require the use of an active manually operated depth control system, and they generally require the use of divers to detach the vehicle from the barge during deployment and to re-attach the vehicle during retrieval. Personnel are also generally required to operate the active depth control system during launch and retrieval.
A submersible launch, recovery and transport vehicle (LRT) of the aforementioned type was developed for transporting and deploying research submarines in rough waters in connection with the Hawaii Undersea Research Laboratory (HURL). The LRT is a twin-hulled, submersible platform upon which a submarine may be positioned and secured. It is towed on the surface by a support vessel to a desired dive site, and the LRT and submarine are both submerged, under the control of a diver pilot, to maintain a stable hover at a depth of 50-60 feet. The submarine is released from the platform by diver(s), and the LRT maintains at hover awaiting return of the submarine, or returns to the surface. While this system allows subsea launch and retrieval of submersible vehicles from a sub-sea location that is isolated from surface conditions (waves, etc.), it requires significant assistance in terms of personnel, and any failure of the active depth control system may result in damage to, or loss of the LRT or its payload.
Systems for submerging work platforms and for supporting submerged work platforms during underwater activities are also known. In one system disclosed in U.S. Pat. No. 5,507,596, a support system for supporting a submerged work platform using one or more vessels uses a plurality of cables connected between the surface support vessel(s) and the underwater platform. Several individually controlled cables are used to provide a desired number of degrees of freedom of control vis a vis the work platform. The surface vessel(s) (e.g., ship(s) or barge(s)) are subject to surface sea motions, and the motions of the support structure(s) are sensed and the length of the cables is actively adjusted to maintain the work platform stationary, even as the support vessel(s) move at the sea surface.
Notwithstanding the existence of various launch and retrieval systems, and of various schemes for supporting underwater platforms, there remains a need for a simple vessel and payload launch and retrieval system that permits the safe deployment and retrieval of equipment and vessels in heavy sea conditions, and that does not require substantial vessel or personnel support or active depth control systems.